Digital images are typically transmitted to an output device for consumption. Output devices convert a digital image to a format that can be viewed or otherwise perceived by a user. Common output devices include, but are not limited to, printers, plotters, and displays. There are many types of printers including inkjet, bubble-jet, laser and others. Many of these printers convert colored elements in a digital image to one or more elements on output media. This can be achieved by converting image elements to ink dots or toner dots applied to a printed page. Various colors are achieved by mixing or overlaying a variety of inks or toners of different colors. Many printers use three ink or toner colors consisting or cyan, magenta and yellow as well as a black ink or toner.
Display devices may also be used as output devices for digital images and as an input device. Displays may include cathode-ray tube (CRT) devices, liquid crystal display (LCD) devices, light-emitting diode (LED) devices, gas-plasma devices and many others. Display devices usually use a three-color pixel arrangement wherein red, green and blue (RGB) sub-pixels are used to create a various color elements in an image display. The luminance or some other attribute of each colored sub-pixel may be varied to achieve a wide variety of colors.
Many other mechanisms and devices may be used to output and display digital images, however due to the nature of each type of device, each one has limitations in the range of colors that it can output or display. For example, the specific ink colors and the quantity of inks used by a device can limit its output to a specific range of colors. The media on which output is generated can also affect output color. The specific nature of each pixel in a display device, such as the diode or phosphor composition and the lighting of a display can limit the range of colors displayed on these devices.
Due to these and other output device limitations, each output device is limited to a specific color range or gamut. This output device gamut may not coincide with the range of colors recorded in a digital image.
Digital images may be recorded through the use of a variety of input devices, such as scanners or digital cameras. These devices typically use a coupled-charge device (CCD) or a similar device that records an array of data that represents color and intensity values for a matrix of image elements. The nature of the scene being photographed or original being scanned, together with that of the input device, will dictate the color range or gamut of the digital image itself.
Due to the different nature of input and output devices and the many different types of devices, the gamut of the input device rarely matches the gamut of the output device. When the output device gamut does not precisely match the image gamut, the image must be rendered in a way that maps each color in the image to a color within the gamut of the output device. This process may be referred to as gamut mapping.
A display may be considered an “input” device from the viewpoint of gamut mapping. When trying to make a printed rendering of an image that we see on a display, this is the common case. Often, there are colors that can be displayed by the monitor but can't be printed.
Gamut mapping may be achieved by a process called “clipping.” Various techniques may be used for clipping, but, generally, a color that falls outside the output device gamut will be mapped to a color on the surface of the output device gamut.
Another known method of gamut mapping is referred to as “compressing” the image gamut. Again, various methods may be used, but, typically, when the gamut of an image is larger than the gamut of the output device, the entire range of a particular image color or hue is compressed into the range of values producible by the output device. This may render relative image gradations better than clipping, but may do so at a cost of decreased color saturation.